Preconditioning and pressure contours
 

Dear Friends,

I am developing a preconditioned solver for inviscid and viscous flows. I am using the Weiss-Smith preconditioner with Roe scheme for the simulations. In tesing the code with inviscid flows past a bump (Ni's classical test case) and the flow past a cylinder, I found that the pressure contours were quiet noisy for M=0.0005, but the velocity contours were good. However, the pressure and velocity contours for the higher mach numbers like M=0.005 and 0.05 were good. The problem is noise in pressure contours when the mach number goes of the order of 1e-4 or below, though the velocity contours and surface pressure distributions are really good.

Any advice and suggestions will be appreciated

Thanks in advance

Ganesh

Re: Preconditioning and pressure contours
 

Dear Ganesh,

For preconditioned Roe scheme, dissipation terms are modified only. I hope you are OK with this. You must obtain smooth pressure (and other flow variables)distributions at mach numbers down to 10^-6. For external flows, the use of modified form of characteristic BC are essential. However, modified non-characteristic BCs work very well as well (if BC are placed far away from the model (~20 chords)). Turkel's article has a simple form for BCs in case of both external/internal flows.

Regards.

Re: Preconditioning and pressure contours
 

This is an indication that preconditioning is not working at the lower mach numbers.

You can try the preconditioned roe flux given by viozat

http://citeseer.ist.psu.edu/312585.html

They also give a different perspective on preconditioning in terms of asympotic behaviour of compressible flows at low mach numbers. Examples of noisy pressure contours are also given in this paper.